Multiple frequency grain moisture sensor for combines

ABSTRACT

A grain moisture sensing system having an excitation signal source for producing an excitation signal. A sensor cell having a driven plate and a sense plate that applies to the excitation signal for captive measurement between the driven plate and the sense plate to produce a current at the sense plate. Connected to the excitation source is a first synchronous detector and connected to the sense plate is a second synchronous detector.

CROSS REFERENCE TO A RELATED APPLICATION

This application is a division of U.S. application Ser. 10/718,147 filedNov. 20, 2003 now U.S. Pat. No. 6,917,206, which is a division of Ser.No. 10/145,463 filed May 14, 2002, now U.S. Pat. No. 6,647,653 and is adivision of 10/003,884 file Oct. 25, 2001, now U.S. Pat. No. 6,686,749issued Feb. 3, 2004.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates generally to grain moisture sensors. Morespecifically, the present invention relates to an improved grainmoisture sensor for combines.

2) Related Art

Grain moisture sensors have been used in combines, particularly inprecision agriculture applications. Continuous or instantaneous grainmoisture readings allow an operator to observe the moisture of the grainas it is being harvested. In conjunction with a GPS unit, a moisturesensor can be used to provide moisture mapping. In addition, moisturesensors are used in yield monitoring applications. When used incombination with a grain flow sensor, the moisture sensor information isoften used to calculate the number of dry bushels in a field and thenumber of bushels per acre based on the number of wet bushels and themoisture content.

Moisture sensors in combines are commonly mounted in one of two places.The first of these places is in the grain tank auger. The grain tankauger is also known as the loading auger in a combine. There are anumber of problems with mounting the moisture sensor in this location.The first is that in order to mount the moisture sensor the flighting ofthe loading auger must be removed. With removed flighting, material canbuild up which requires the operator to clean the sensor. If themoisture sensor is not kept clean, readings may be inaccurate or themoisture sensor may be inoperable.

A further problem with mounting the moisture sensor in the loading augerof a combine is the lag time or delay encountered when measuringmoisture. When the moisture sensor is mounted in the loading augerposition, moisture sensor readings are not taken until the grain isactually in the loading auger of the combine. Therefore, grain musttravel up the elevator and fill the sump of the transition housingbefore the auger is able to deliver grain to the sensor and a moisturemeasurement can be taken. This deficiency frustrates the use of amoisture sensor in precision agriculture applications, making it moredifficult to correctly associate a particular field location with aparticular grain moisture.

A further problem with mounting grain moisture sensors in a loadingauger is that such a moisture system does not provide for determiningwhen there is sufficient grain present for a grain moisture measurement.Grain moisture sensors usually include capacitive plates. The volumebetween the plates must be covered before an accurate grain moisturemeasurement can be made. A moisture sensor that is not filled with grainis not accurately measuring the moisture of the grain. Therefore, thisinability to know when the capacitive plate is covered can result inerroneous grain moisture measurements.

Another location that has been used to mount grain moisture sensors ison the side of the clean grain elevator. The clean grain elevatormounting location is thought to provide a steadier flow of grain.Further, the clean grain elevator location may avoid causing acceleratedwear of the auger assembly and does not obstruct grain flow in themanner which the loading auger location may. Despite these improvements,a number of problems remain with mounting a moisture sensor on the sideof the clean grain elevator in a combine. One problem relates to theslow cycle time of the moisture sensor. In a low flow condition which isnot uncommon in grain harvesting, the sensor can be extremely slow tofill. For example, it may take up to four minutes to fill the sensor.Therefore, the number of moisture sensor readings is reduced and themoisture sensor data is insufficient for providing accurate measurementsfor moisture maps, yield determinations, and other purposes.

A further problem with mounting moisture sensors on the side of theclean grain elevator relates to the sensitivity of this mountinglocation in the presence of side slopes. It is not uncommon for acombine to be operating on a hill or slope. When the combine is operatedon a slope such that the grain flow is directed away from the moisturesensor inlet, it is nearly impossible to fill the grain moisture sensorwith sufficient grain to make a moisture determination.

A further problem with mounting moisture sensors on the clean grainelevator relates to grain leaks. When mounted on the side of the cleangrain elevator, any grain leaks that occur result in the leaking grainspilling on the ground, as the grain leaks are not contained.

Another problem in grain moisture sensing relates to the sensor cell.Typically, the sensor cell consists of a parallel plate capacitor inwhich the grain serves as the dielectric material. The cell capacitanceand therefore the permittivity of the grain between the plates ismeasured. From this measurement, the moisture of the grain isdetermined. Normally, these cell designs are not as close to an idealparallel plate capacitor as desired. In particular, prior art designsfor grain moisture sensors for use in combines use cells that aresubject to electric field fringe effects. A fringe effect occurs whenelectric field lines are not both straight and perpendicular to theplates of the parallel plate capacitor. These fringe effects produce anuncontrollable influence on the measurements from material other thangrain that is close to the cell but outside of the cell. Another problemwith cell designs is that they do not produce uniformly dense electricfield lines between the parallel plates. The nonuniform electric fielddensity creates the problem of unequal sensitivity to grain throughoutthe cell. Thus the measurements of the moisture of the grain within thecell are not as accurate as desired in these respects.

Another problem relating to the prior art relates to the method formeasuring cell capacitance. Measuring the capacitance of a cell filledwith grain is a traditional way of obtaining grain moisture. There aretwo common prior art methods for measuring cell capacitance. The firstmethod is to sense the changes in frequency of a variable oscillatorthat uses cell capacitance as one of its frequency determining elements.The second method is to excite the cell capacitance with a signal havinga known frequency and to measure the absolute value of the resultingcell current, usually with a bridge type of circuit and a peak detector,and then to calculate the capacitance of the cell. Both of these methodstend to be dependent on grain cell construction and are sensitive tonoise, changes in circuit characteristics, and parasitic effects. Thefirst method also has the problem of poor control of frequency,especially as moisture varies. Both of these methods are also singledimensional, lacking the ability to measure both the dielectric and theloss properties of the grain. Therefore numerous problems remain withthis type of sensing.

The combination of the dielectric and loss parameters is known as thecomplex permittivity. Complex permittivity is an intrinsic, frequencydependent material property. The knowledge of the grain's complexpermittivity at more than one frequency has been found to be a part ofadvanced moisture level assessment as has been demonstrated by USDAstudies. Despite this observation, problems remain.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a grainmoisture sensor for use on a combine that improves upon the state of theart.

It is another object of the present invention to provide a grainmoisture sensor that provides accurate and consistent grain moisturemeasurements.

It is a further object of the present invention to provide a grainmoisture sensor that does not require the removal of flighting in theloading auger for cleaning.

Yet another object of the present invention is to provide a grainmoisture sensor that avoids lags in time between when grain is harvestedand when the moisture measurement is taken.

A further object of the present invention is to provide a grain moisturesensor that is capable of determining when the sensor cell is full.

Yet another object of the present invention is to provide an improvedgrain moisture sensor that is less affected by low flow conditions.

Yet another object of the present invention is to provide a grainmoisture sensor for use in a combine that is insensitive to changes inthe side slope of the ground being harvested.

Yet another object of the present invention is to provide a grainmoisture sensor that contains grain leaks.

A still further object of the present invention is to provide a grainmoisture sensor with a cell that has characteristics closer to an idealparallel plate capacitor.

A still further object of the present invention is to provide a grainmoisture sensor that provides for uniform electric field density toallow for equal sensitivity to grain throughout the cell.

Yet another object of the present invention is to provide a grainmoisture sensor with a cell for reducing fringe effects produced bymaterial other than grain that may be close to, but outside of the cell.

Yet another object of the present invention is to provide a grainmoisture sensor that provides for increased protection fromelectromagnetic interference.

A still further object of the present invention is to provide a grainmoisture sensor that provides for the measurement of complexpermittivity of the grain.

Another object of the present invention is to provide a grain moisturesensor that provides for the measurement of complex permittivity of thegrain at more than one frequency.

A grain moisture sensor of the present invention provides for thesensing of the moisture of grain being harvested by a combine. Oneaspect of the present invention relates to the location of the grainmoisture sensor on the combine. According to the present invention, thegrain moisture sensor is mounted off of the front of the clean grainelevator transition housing inside of the grain tank. This provides theadvantages of access to the grain moisture sensor if required and theadvantage that all leaks are contained. A further advantage is that thegrain moisture sensor fills positively with grain. Further, thislocation of the grain moisture sensor allows for the sensor to always befilled regardless of the slope conditions of the combine.

Another aspect of the present invention relates to the cell design ofthe sensor. The cell of the present invention includes a driven plate towhich excitation voltages are applied, a sense plate proximate andparallel to the driven plate for measuring current that passes throughthe cell, a fill plate adjacent to the sense plate for determining whenthe cell is full, and a guard adjacent to the sense plate and the fillplate for protecting the sense plate and the fill plate. The guard iselectrically isolated from, but at the same potential as a sensed plate.The guard is parallel to and dimensionally larger than the sense platein order to shape the electric field. The presence of the guard plateprovides the advantage of straight electric field lines perpendicular tothe sense plate and of uniform density throughout the region between theparallel plates. This results in reduced fringe effects and uniformelectric field density allowing for equal sensitivity to grainthroughout the cell. In addition, the guard shields the sense plate fromexternal electric fields generated by sources other than the drivenplate. The fill plate provides the advantage of accurate determinationof whether or not the cell is full.

A further aspect of the present invention is the method in which thecapacitance of a cell filled with grain is measured. The presentinvention provides for measurement of the complex permittivity of thegrain. Further, the present invention provides for measurement of thecomplex permittivity at more than one frequency. This provides theadvantage of permitting compensation for variations in grain density andconductivity effects which is particularly important in mobile moisturesensing applications such as the use of a moisture sensor on a combine.According to this aspect of the present invention, the circuit measuresthe real and imaginary components of both the excitation voltage and thesense current. From these values, the complex admittance of the cell iscalculated. The measurements are repeated for the empty cell and thecell filled with grain. When the empty cell is not available, thecalibrated reference admittances are used instead. The grain complexpermittivity can then be calculated from these measurements. Mixers areused in the measurement of real and imaginary components of the voltageand current. This synchronous detection method has a very narrow bandfiltering effect, greatly reducing noise influence on the measurement. Avirtual ground method of measuring low-level currents is used to providethe advantage of a substantial reduction in the influence of parasiticelements at the current sensing node. In addition, measurements can becorrected with the calibrated references to compensate for anyenvironmental changes that may influence the circuit characteristics.This provides the advantage of securing stable and repetitive results.

In this matter, the present invention provides advantages in an improvedgrain moisture sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side view that shows a combine with a grain moisture sensoraccording to the present invention.

FIG. 1B is a side view of the grain moisture sensor of the presentinvention mounted in a combine and in a fill position.

FIG. 1C is a side view of the grain moisture sensor of the presentinvention mounted in a combine in a sensing position.

FIG. 2 is a side cross section of the cell of the grain moisture sensorof the present invention.

FIG. 3 is a side cross section of the cell of FIG. 2 showing theequipotential lines of the electric field that is created when anexcitation voltage is applied to the driven plate of the presentinvention.

FIG. 4 is a circuit schematic of a model for the capacitor cellaccording to the present invention.

FIG. 5 is a block diagram of the admittance measuring circuit accordingto the grain moisture sensor of the present invention.

FIGS. 6A and 6B are block diagrams of the moisture sensor circuitaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a combine having a grain moisture sensor according to thepresent invention. In FIG. 1A, the combine 2 is shown with a grain tank10. In addition, the clean grain elevator 4 is shown. Grain from theclean grain elevator 4 travels to the transition housing 15 of the graintank 10. The sump 6 of the transition housing is also shown.

FIGS. 1B and 1C show side views of the grain moisture sensor of thepresent invention as mounted in a combine. The grain tank 10 shownincludes the grain moisture sensor 12. The grain moisture sensor 12 islocated in the grain tank 10 of the combine near the mass flow sensor11. The cell opening 16 is located below the impact plate 14 in thetransition housing 15. Although an impact plate 14 is shown, the presentinvention contemplates that other deflectors can be used. In thislocation, the cell 13 is positively filled due to the direct or indirectvelocities of grain created from the clean grain elevator paddles (notshown). This permits the cell 13 to be filled at high rates. Thisreduces any problems with slow cycle times associated with low flowconditions as here, the cell 13 is filled at a high rate due to itsplacement within the stream of grain created by the clean grainelevator. The cell 13 is placed in line with the cell inlet. Aplunger/piston 18 with an electric actuator is used to force the grainsample back out of the inlet opening. In FIG. 1C, the cell 13 andplunger/piston 18 are in a sensing position.

FIG. 2 illustrates a side view of the cell 13 of the present invention.The cell opening 16 or spacing is filled with grain. On either side ofthe spacing are parallel capacitor plates 64 and 68. The driven plate 64is the plate to which an excitation voltage is applied. The sense plate68 is the plate at which current is measured passing through the cell.The fill plate 66 is adjacent to the sense plate 68 and parallel to thedriven plate 64.

The fill sensor plate is one-fourth the size of the sense plate. Todetermine when the cell is full, the fill plate should indicate ameasured reading of one-fourth the sense plate measured reading.Although in this embodiment the fill sensor is one-fourth the size ofthe sense plate, the present invention contemplates numerous variationsin the sizes of the plates. This is merely one example of a relativesize which is convenient and useful.

The guard 70 is strategically placed behind the sense plate 68 and thefill plate 66. The guard 70 is parallel to and dimensionally larger thanthe sense plate 68 in order to shape the electric field. In addition,the guard 70 also shields the sense plate from external electric fieldsgenerated by sources other than the driven plate 64.

FIG. 3 illustrates the cell of the moisture sensor with theequipotential lines of the electric field that is generated when anexcitation signal is applied to the driven plate 64. Due to theplacement of the shield or guard 70, which is electrically isolated frombut at the same potential as the sense plate 68, the effect on theelectric field lines in the vicinity of the sense plate is to producethe equivalent of an ideal parallel plate capacitor without fringeeffects. The electric field lines are straight in nature andperpendicular to the sense and fill plates. Further, the electric fieldlines are uniform in density throughout the region between the parallelplates. The result is that fringe effects are reduced. Fringe effectsproduce uncontrollable influence on the measurements from material otherthan the grain that are close to but outside of the cell. Here, thestraight electric field lines within the cell show that the cell islargely immune from that influence. Further, the uniform electric fielddensity provides for equal sensitivity to grain throughout the entirecell. In addition, the entire cell and the electronics are contained ina metal enclosure 60. The metal enclosure 60 serves as anelectromagnetic interference shield, further isolating the entire cellfrom other sources of electromagnetic energy.

The present invention provides for grain moisture calculations based onthe measurement of the complex relative permittivity of the grain(henceforth referred to as “complex permittivity”). FIG. 4 illustrates aschematic diagram of a circuit that is electrically equivalent to thecapacitor cell of the present invention. This equivalent circuitincludes an ideal capacitor 82 having a value of C in parallel with anideal resistor 92 having a value of R. The ideal capacitor 82 representsthe capacitive or energy storing property of the cell and the idealresistor 92 represents the conductive or energy dissipating property ofthe cell. C and R are dependent on the frequency of excitation and onthe moisture, temperature, and certain other properties of the grain.

The complex admittance of the cell is$Y = {\frac{1}{R} + {j \cdot \omega \cdot C}}$

-   -   where        -   ω=2·π·F        -   f=frequency of excitation        -   j=the imaginary unit

When the cell is empty it has essentially no energy dissipatingproperties. Its admittance is very close to that of an ideal capacitorhaving a value of C_(CE):Y _(CE) =J·ω·C _(CE)

When the cell is filled with grain it has both energy dissipating andenergy storing properties. Its admittance is$Y_{CF} = {\frac{1}{R_{CF}} + {j \cdot \omega \cdot C_{CF}}}$Dividing the filled cell admittance by the empty cell admittance gives$\begin{matrix}{\frac{Y_{CF}}{Y_{CE}} = {\frac{\frac{1}{R_{CF}} + {j \cdot \omega \cdot C_{CF}}}{j \cdot \omega \cdot C_{CE}} = {\frac{\frac{1}{R_{CF}}}{j \cdot \omega \cdot C_{CE}} + \frac{C_{CF}}{C_{CE}}}}} \\{= {{\frac{C_{CF}}{C_{CE}} - {j \cdot \frac{1}{\omega \cdot C_{CE} \cdot R_{CE}}}} == ɛ}}\end{matrix}$This ratio is the complex permittivity of the grain. Complexpermittivity is an intrinsic material property, dependent only on thefrequency of excitation and on the moisture, temperature, and certainother properties of the grain. It is independent of the dimensions andshape of the cell. Complex permittivity is commonly written as follows:ε=ε′−j·ε″

-   -   where        -   ε′=dielectric constant        -   ε″=loss factor

It is an objective of the circuitry of the present invention to measurethe empty cell admittance and the full cell admittance in order to usethe above equations to compute the complex permittivity of the grain. Asshown on FIG. 4, the complex excitation voltage, V_(C), is appliedacross the circuit. The resulting complex current, I_(C), flows throughthe circuit. V_(C) has real component V_(r) and imaginary componentV_(i):V _(C)=V_(r) +j·V _(i)Ic has real component I_(r) and imaginary component I_(i):I _(C) =I _(T) +j·I _(i)By measuring V_(r), V_(i), I_(r), and I_(i), the complex admittance Ycan be calculated using complex arithmetic:$Y = {\frac{I_{C}}{V_{C}} = \frac{I_{r} + {j \cdot I_{i}}}{V_{r} + {j \cdot V_{i}}}}$

FIG. 5 illustrates the admittance measuring circuit used in the grainmoisture sensor according to the present invention. In FIG. 5, theadmittance measuring circuit 100 is shown. For explanation purposes, thefollowing definitions are used:

-   U_(R1m) is defined as the measured value of U_(R1) defined below.-   U_(R1m1) is the real part (in phase signal value) of U_(R1m)-   U_(R1m2) is the imaginary part (quadrature signal value) of U_(R1m).-   U_(R2m) is defined as the measured value of U_(R2) defined below.-   U_(R2m2) is the real part (in phase signal value) of U_(R2m)-   U_(R2m2) is the imaginary part (quadrature signal value) of U_(R2m).-   U_(Cm) is defined as the measured value of U_(C) defined below.-   U_(Cm1) is the real part (in phase signal value) of U_(Cm)-   U_(Cm2) is the imaginary part (quadrature signal value) of U_(Cm).-   W_(R1m) is defined as the measured value of W_(R1) defined below.-   W_(R1m1) is the real part (in phase signal value) of W_(R1m)-   W_(R1m2) is the imaginary part (quadrature signal value) of W_(R1m).-   W_(R2m) is defined as the measured value of W_(R2) defined below.-   W_(R2m2) is the real part (in phase signal value) of W_(R2m)-   W_(R2m2) is the imaginary part (quadrature signal value) of W_(R2m).-   W_(Cm) is defined as the measured value of W_(C) defined below.-   W_(Cm1) is the real part (in phase signal value) of W_(Cm)-   W_(Cm2) is the imaginary part (quadrature signal value) of W_(Cm).-   U_(C) is defined as a complex voltage value that represents the    current passing through the cell.-   U_(R1) is defined as a complex voltage value that represents the    current passing through the first reference.-   U_(R2) is defined as a complex voltage value that represents the    current passing through the second reference.-   W_(C) is defined as a complex voltage value that represents the    voltage across the cell.-   W_(R1) is defined as a complex voltage value that represents the    voltage across the first reference.-   W_(R2) is defined as a complex voltage value that represents the    voltage across the second reference.-   I_(C) is the complex current passing through the cell.-   I_(R1) is the complex current passing through the first reference.-   I_(R2) is the complex current passing throughout the second    reference.-   V_(C) is the complex voltage across the cell.-   V_(R1) is the complex voltage across the first reference.-   V_(R2) is the complex voltage across the second reference.-   Y_(C) is the complex admittance of the cell.-   Y_(R1) is the complex admittance of the first reference.-   Y_(R2) is the complex admittance of the second reference.-   H is the transfer function of the circuitry that performs complex    current measurements.-   G is the transfer function of the circuitry that performs complex    voltage measurements.-   V_(S) is the generated source voltage.-   A_(C) is the transfer function for cell drive voltage.-   A_(R) is the transfer function for reference drive voltage.-   D is the transfer function of the phase and gain mismatch between    the measured real (in-phase) and measured imaginary (quadrature)    components of the complex current and voltage. This mismatch is    caused by imperfections in the circuit elements that do the    measuring. D is also known as the “mixer transformation matrix”. It    is an object of the present invention to measure the value of D and    to correct for its influence.

In the admittance measuring circuit 100, a generated source voltage 102(V_(S)) is selectively applied to the cell or to one of a plurality ofreferences through an associated transfer function as indicated byreference numerals 104, 106, and 108. When V_(s) is applied to transferfunction A_(C) 104, a voltage V_(C) is produced which is applied to thecomplex admittance for the cell, Y_(C) 110. Similarly, when the voltage102 (V_(S)) is applied to a first transfer function A_(R) 106 theresulting voltage V_(r1) is applied to the complex admittance of thefirst reference admittance, Y_(R1) 112, and when the signal 102 isapplied to the second transfer function A_(r) 108, the resulting voltageV_(R2) is applied to the second complex admittance 114 (Y_(R2)). Each ofthe resulting currents is summed in an adder 116. Where only one path isselected, only one of these signals will be nonzero. The resultingcurrent is then I_(C) if the cell is selected, I_(R1) if the firstreference is selected, and I_(R2) if the second reference is selected.The resulting current flows through a circuit having transfer function H120, H being a transfer function for converting complex current to acomplex voltage for measurement purposes. The resulting voltage measuredthrough node 121 represents the complex current through either the celladmittance or one of the reference admittances. The real and imaginary(in-phase and quadrature) components of this voltage are determined byapplying the voltage to the subcircuit consisting of blocks 128, 129,and 130 as shown in FIG. 5. Thus in this manner, voltages U_(Cm1) andU_(Cm2) representing the complex current through the cell are measured.By selecting either of the references, voltages representing the complexcurrent through the first reference or through the second reference canalso be measured.

In addition to measuring voltages that represent the complex currentvalues, voltages that represent the complex voltage values are alsocalculated according to the circuit. The voltages from the cell, V_(C),the first reference, V_(R1), and the second reference, V_(R2) areapplied to an adder 118. As only one of the references or the cell isselected at a time, only one of these values will be non-zero. Theresult is applied to a transfer function 126 resulting in a complexvoltage at node 127. The real and imaginary (in-phase and quadrature)components of this voltage are determined by applying the voltage to thesubcircuit consisting of blocks 128, 129, and 130 as shown in FIG. 5.

In this manner, the circuit shown in FIG. 5 provides for determinationof the real and imaginary parts of both the voltage and the currentassociated with a particular admittance. This admittance being eitherthat associated with the cell of the grain moisture sensor or thatassociated with one of the reference admittances of the grain moisturesensor.

To further explain, the following mathematical relationships arepresent: $G = \frac{W}{V}$ $H = \frac{U}{I}$

In each case, the respective transfer functions are defined as the ratioof the output of the function to the input of the function.

In addition, the admittance is defined mathematically as:$Y = {\frac{I}{V} = {\frac{\frac{U}{H}}{\frac{W}{G}} = {\frac{U}{W} \cdot \frac{G}{H}}}}$

Given these general relationships, the admittance of a reference isdefined as: ${YR} = {\frac{U_{R}}{W_{R}} \cdot \frac{G}{H}}$

Further, the empty cell admittance, YCE, and a full cell admittance,YCF, are calculated as follows:$Y_{CE} = {\frac{U_{CE}}{W_{CE}} \cdot \frac{G}{H}}$$Y_{CF} = {\frac{U_{CF}}{W_{CF}} \cdot \frac{G}{H}}$

If the measurements for the reference admittance and the cell admittanceare done in the same environmental conditions, it can be assumed thatboth G and H are the same in the cell admittance equations and thereference admittance equations. Then the following characterizes theempty cell and reference calibration factor F:${\frac{W_{R}}{U_{R}} \cdot Y_{R}} = {\left. {\frac{W_{CE}}{U_{CE}} \cdot Y_{CE}}\Rightarrow F \right. = {{\frac{W_{CE}}{U_{CE}} \cdot \frac{U_{R}}{W_{R}}} = \frac{Y_{R}}{Y_{CE}}}}$

The reference calibration factor, F, gives the ratio of the referenceadmittance to the empty cell admittance at the same environmentalconditions. Thus a reference admittance can be used instead of an emptycell admittance for calibration purposes.

Assuming that F will stay constant, the sampled grain's complexpermittivity can be calculated as:$ɛ = {\frac{U_{CF} \cdot W_{R}}{W_{CF} \cdot U_{R}} \cdot F}$

-   -   Where:        ε=ε′−jε″

Thus, the present invention provides for measurement of the complexpermittivity of grain for moisture sensing purposes.

To make accurate current and voltage measurements it is necessary thatthe in-phase (IP) and quadrature (Q) local oscillator signals used withmixers 216, 220, and 224 to extract the real and imaginary components ofcomplex signals have a phase difference of exactly 90 degrees and haveidentical amplitudes at their fundamental frequencies. Errors will beintroduced to the extent that this is not the case. By using tworeference admittances of known and stable values however, corrections tothese errors are made.

The D functions 124 and 130 represent the distortion of the imaginarypart with the respect to the real part of all measured complex values.All measured values U_(m) and W_(m) can be corrected, using the sameformula to obtain U and W, which are the values before any measurementdistortion error is introduced.

The following is the distorted relationship between the complex voltagesrepresenting cell and reference currents and their measured values:U=[1 j]·D ⁻¹ ·U _(m)

-   -   where: $D^{- 1} = {{PFC} = \begin{bmatrix}        1 & 0 \\        {pfc1} & {pfc2}        \end{bmatrix}}$ $U_{m} = \begin{bmatrix}        U_{m1} \\        U_{m2}        \end{bmatrix}$

The same distorted relationship holds between the complex voltagesrepresenting cell and reference voltages and their measured values:W=[1 j]·D ⁻¹·W_(m)

-   -   where: $D^{- 1} = {{PFC} = \begin{bmatrix}        1 & 0 \\        {pfc1} & {pfc2}        \end{bmatrix}}$ $W_{m} = \begin{bmatrix}        W_{m2} \\        W_{m2}        \end{bmatrix}$        Expanding the above equations gives        U = U_(m1) + j ⋅ (pfc1 ⋅ U_(m1) + pfc2 ⋅ U_(m2))        W = W_(m1) + j ⋅ (pfc1 ⋅ W_(m1) + pfc2 ⋅ W_(m2))

The pfc1 and pfc2 correction factors are found through the use of twodifferent references having known and stable admittance values ofdifferent phase angles. As an example, in one embodiment of the presentinvention the first reference is a temperature stable 1% capacitor (COG)with a value of 15 pF (admittance Y_(R1)) and the second reference is aprecision 0.1% resistor with value of 2000 Ω (admittance Y_(R2)). Otherreference values may be used as well.

The ratio of the reference admittances is computed as follows, with theexample values also shown:$R = {\frac{Y_{R1}}{Y_{R2}} = {{Q_{R} + {j \cdot Q_{1}}} = {2000 \cdot j \cdot \left( {2{\pi \cdot f \cdot 15 \cdot 10^{- 12}}} \right)}}}$The ratio of the raw measurements of two references is:${Rm} = \frac{U_{R1} \cdot W_{R2}}{W_{R1} \cdot U_{R2}}$Expanding the above equation to include the measured values results in:${Rm} = \frac{\begin{matrix}{\left( {U_{R1m1} + {j\left( {{U_{R1m1} \cdot {pfc1}} + {U_{R1m2} \cdot {pfc2}}} \right)}} \right) \cdot} \\\left( {W_{R2m1} + {j\left( {{W_{R2m1} \cdot {pfc1}} + {W_{R2m2} \cdot {pfc2}}} \right)}} \right)\end{matrix}}{\begin{matrix}{\left( {W_{R1m1} + {j\left( {{W_{R1m1} \cdot {pfc1}} + {W_{R1m2} \cdot {pfc2}}} \right)}} \right) \cdot} \\\left( {U_{R2m1} + {j\left( {{U_{R2m1} \cdot {pfc1}} + {U_{R2m2} \cdot {pfc2}}} \right)}} \right)\end{matrix}}$R_(m) is set equal to R and two quadratic equations in two unknowns(pfc1, pfc2) are derived:a ₁ ·pfc ₁ ² +b ₁ ·pfc ₂ ² +c ₁ ·pfc ₁ ·pfc ₂ +d ₁ ·pfc ₁ +e ₁ ·pfc ₂ +f₁=0 (from real part)a ₂ ·pfc ₁ ² +b ₂ ·pfC ₂ ² +c ₂ ·pfc ₁ ·pfc ₂ +d ₂ ·pfc ₁ +e ₂ ·pfc₂+f₂=0 (from imaginary part)

-   -   where:        a ₁ =Q _(R) ·W _(R1m1) ·U _(R2m1) −U _(R1m1) ·W _(R2m1)        a ₂ =Q _(I) ·W _(R1m1) ·U _(R2m1)        b ₁ =Q _(R) ·W _(R1m2) ·U ^(R2m2) −U _(R1m2) ·WR2m2        b₂ =Q _(I) W _(R1m2) ·UR2m2        c₁ =Q _(R) ·W _(R1m1) ·U _(R2m2) +Q _(R) ·W _(R1m2) ·U _(R2m1)        −U _(R1m1) ·W _(R2m2) −U _(R1m2) ·W _(R2m1)        c ₂ =e ₁        d ₁=2·Q _(I) ·W _(R1m1) ·U _(R2m1)        d ₂=−2·a ₁        e ₁ =Q _(I)·(W _(R1m1) ·U _(R2m2) +W _(R1m2) ·U _(R2m1))        e ₂ =−c ₁        f ₁ =−a ₁        f ₂ =−a ₂

These two quadratic equations are then solved simultaneously for pfc1and pfc2. As a simple closed form solution is not available, they may besolved by Newton-Raphson iteration for example. Other numerical equationsolving algorithms may be used as well. The solution is known to be nearthe point (pfc1=0, pfc2=1) hence this is preferably used for a startingpoint. In theory four different solutions are possible. Any solution notnear (0,1) shall be considered extraneous. In a software implementation,an appropriate error condition can be set. This is not likely to happen,however, if it does occur, precautions can be taken when the errorcondition is present.

FIGS. 6A and 6B show a schematic of the grain moisture sensor accordingto the present invention. The schematic shows a number of input andoutput lines for connection to an intelligent control such as aprocessor, microcontroller, integrated circuit, or other device. Thisschematic shows merely one circuit configuration of the presentinvention. The present invention provides for the ability to selectivelymeasure one of a plurality of complex admittances at a plurality offrequencies.

The inputs to the system (outputs from an intelligent controller) areshown in FIG. 6A. The inputs include a first frequency input 164 and asecond frequency input 166. Optionally a first sine wave generator 178and a second sine wave generator 180 are used. The sine wave generatorstake the square wave output of a microcontroller, divide the frequencyas necessary, and smooth the output such that a sinusoidal signal isproduced. The output from the first sine wave generator 178 iselectrically connected to three switch inputs of dual quad switch 198.In addition, the output from the first sine wave generator 178 iselectrically connected to a 90 degree phase shifter 194. The 90 degreephase shifter 194 is constructed such that its output signal is 90degrees out of phase with its input signal. The 90 degree phase shifter194 is electrically connected a switch input of the dual quad inputswitch 198. The output of the second sine wave generator 180 issimilarly connected.

The first sine wave generator 178 and the second sine wave generator 180operate at different frequencies. For example, the first sine wavegenerator 178 operates at 10 MHz while the second sine wave generator180 operates at 1 MHz.

The dual quad input switch 198 is controlled by input 174 and input 172that are used to select one of the signals. One of the outputs from theswitch is electrically connected to an input of the dual quad outputswitch 200. Inputs 168 and 170 are connected to the switch 200 tocontrol which of the outputs is selected. The outputs are buffered andthen electrically connected to the sensor cell 208, a first referenceadmittance 210, and a second reference admittance 212. The referenceadmittances are used for calibration purposes.

As shown in FIG. 6B, the buffered outputs, which drive the cell and thetwo references, are also electrically connected to a summing circuit214. The output from the summing circuit 214 is electrically connected,through high pass filter 215, to a mixer 216. This mixer 216 also has alocal oscillator input electrically connected to an output from theswitch 198 (FIG. 6A). The output of mixer 216 passes through low passfilter 226 and is then electrically connected to an analog-to-digitalconverter and read by the microcontroller. The output of the mixer 216has a DC voltage that is proportional to that component of the inputvoltage signal that is in-phase with the local oscillator.

The sense plate of the sensor cell 208 and the first reference 210 andthe second reference 212 of FIG. 6A are electrically connected to asumming current to voltage converter 218 shown in FIG. 6B. The summingcurrent to voltage converter has a low impedance, virtual ground type ofinput. The output of the summing current-to-voltage converter iselectrically connected, through high pass filter 219, to a second mixer220. The second mixer 220 also has a local oscillator input electricallyconnected to an output from switch 198 (FIG. 6A). The output of mixer220 passes through low pass filter 228 and is then electricallyconnected to an A/D converter and read by the microcontroller. Theoutput of the mixer 220 has a DC voltage that is proportional to thatcomponent of the input signal that is in-phase with the localoscillator.

In addition, the current, I_(F) from the fill plate on the sensor cell208 (shown in FIG. 6A) passes through the current to voltage converter222. This current to voltage converter also has a low impedance, virtualground type of input. The output of the current to voltage converter 222is electrically connected, through high pass filter 223, to a thirdmixer 224. The third mixer 224 also has a local oscillater input that iselectrically connected to an output from switch 198. The output of mixer224 passes through low pass filter 230 and then is electricallyconnected to an analog to digital converter and read by themicrocontroller. This configuration permits monitoring of the admittanceof the fill plate relative to that of the sense plate. When thisrelationship is proportional to the relative sizes of the plates, thenthe sensor cell 208 is considered full of grain.

The synchronous detection method for measuring complex signals throughthe use of a local oscillator, a mixer, and a low pass filter, asdescribed above, has a very narrow band pass filtering effect, greatlyreducing noise influence on the measurement. The virtual ground methodof measuring very low-level currents is used to provide the advantage ofa substantial reduction in the influence of parasitic elements at thecurrent summing and sensing node.

Returning to FIG. 6A, a thermistor or other temperature sensor isattached to the driven plate of the sensor cell 208. This is only oneexample of temperature sensor placement. The temperature sensor may alsobe attached to one of the other plates in the cell. The measurement oftemperature allows moisture calculations to be corrected accordingly.

Thus a detailed schematic for the present invention has been shown anddescribed. That which is shown is merely one embodiment of a designaccording to the present invention. The present invention contemplatesvariations in the frequencies used, the number of references, theparticular electrical components used to perform a particular functionor set of functions, and other variations.

Therefore a novel grain moisture sensor has been disclosed. According toone aspect of the invention, the grain moisture sensor provides for themeasurement of complex admittance at multiple frequencies. According toanother aspect of the invention, the grain moisture sensor is mounted inthe grain tank of a combine. According to another aspect of the presentinvention, the grain moisture sensor is of a sensor cell design thatguards the capacitive plates from fringe effects. According to anotheraspect of the invention, a fill sensor is provided so that accuratedeterminations can be made as to when the sensor cell is full and readyfor measurement.

ASSIGNMENT

The entire right, title and interest in and to this application and allsubject matter disclosed and/or claimed therein, including any and alldivisions, continuations, reissues, etc., thereof are, effective as ofthe date of execution of this application, assigned, transferred, soldand set over by the applicant(s) named herein to Deere & Company, aDelaware corporation having offices at Moline, Ill. 61265, U.S.A.,together with all rights to file, and to claim priorities in connectionwith, corresponding patent applications in any and all foreign countriesin the anem of Deere & Company or otherwise.

1. A grain moisture sensing system comprising: an excitation signalsource for producing an excitation signal; a sensor cell having a drivenplate for applying the excitation signal and a sense plate proximate toand substantially parallel with the driven plate for capacitivemeasurement across a spacing between the driven plate and the senseplate such that a sense current is produced at the sense plate; theexcitation signal source electrically connected to the driven plate ofthe sensor cell; a first synchronous detector adapted for measuringcomponents of the excitation signal, the synchronous detectorelectrically connected to the excitation source; and a secondsynchronous detector adapted for measuring components of the sensecurrent, the second synchronous detector operatively connected to thesense plate.
 2. The grain moisture system of claim 1 wherein the firstsynchronous detector is adapted for alternatively measuring imaginarycomponents of the excitation signal and real components of theexcitation signal.
 3. The grain moisture system of claim 1 wherein thesecond synchronous detector is adapted for alternatively measuringimaginary components of the sense current and real components of thesense current.
 4. The grain moisture system of claim 1 wherein the firstsynchronous detector is a mixer and the second synchronous detector is amixer.
 5. The grain moisture sensor of claim 1 wherein the excitationsignal source is a switch adapted for alternatively selecting one of afirst frequency in-phase signal, a first frequency quadrature signal, asecond frequency in-phase signal, and a second frequency quadraturesignal.